United States Office of Water EPA 832-F-00-014 Environmental Protection Washington, D.C. September 2000 Agency Wastewater Technology Fact Sheet Trickling Filters DESCRIPTION ADVANTAGES AND DISADVANTAGES Trickling filters (TFs) are used to remove organic Some advantages and disadvantages of TFs are matter from wastewater. The TF is an aerobic listed below. treatment system that utilizes microorganisms attached to a medium to remove organic matter Advantages from wastewater. This type of system is common to a number of technologies such as rotating C Simple, reliable, biological process. biological contactors and packed bed reactors (bio- towers). These systems are known as C Suitable in areas where large tracts of land attached-growth processes. In contrast, systems in are not available for land intensive treatment which microorganisms are sustained in a liquid are systems. known as suspended-growth processes. C May qualify for equivalent secondary APPLICABILITY discharge standards. TFs enable organic material in the wastewater to be C Effective in treating high concentrations of adsorbed by a population of microorganisms organics depending on the type of medium (aerobic, anaerobic, and facultative bacteria; fungi; used. algae; and protozoa) attached to the medium as a biological film or slime layer (approximately 0.1 to C Appropriate for small- to medium-sized 0.2 mm thick). As the wastewater flows over the communities. medium, microorganisms already in the water gradually attach themselves to the rock, slag, or C Rapidly reduce soluble BOD5 in applied plastic surface and form a film. The organic wastewater. material is then degraded by the aerobic microorganisms in the outer part of the slime layer. C Efficient nitrification units. As the layer thickens through microbial growth, C Durable process elements. oxygen cannot penetrate the medium face, and anaerobic organisms develop. As the biological C Low power requirements. film continues to grow, the microorganisms near the surface lose their ability to cling to the medium, C Moderate level of skill and technical and a portion of the slime layer falls off the filter. expertise needed to manage and operate the This process is known as sloughing. The sloughed system. solids are picked up by the underdrain system and transported to a clarifier for removal from the wastewater. Disadvantages organic loads better than cross flow media. The plastic medium also required additional provisions, C Additional treatment may be needed to meet including ultraviolet protective additives on the top more stringent discharge standards. layer of the plastic medium filter, and increased plastic wall thickness for medium packs that are C Possible accumulation of excess biomass that installed in the lower section of the filter where cannot retain an aerobic condition and can loads increase. impair TF performance (maximum biomass thickness is controlled by hydraulic dosage rate, type of media, type of organic matter, temperature and nature of the biological growth). C Requires regular operator attention. C Incidence of clogging is relatively high. C Requires low loadings depending on the medium. C Flexibility and control are limited in comparison with activated-sludge processes. C Vector and odor problems. C Snail problems. DESIGN CRITERIA A TF consists of permeable medium made of a bed Source: Metcalf & Eddy, Inc. and Tchobonaglous, 1998. of rock, slag, or plastic over which wastewater is distributed to trickle through, as shown in Figure 1. FIGURE 1 TYPICAL TRICKLING FILTER Rock or slag beds can be up to 60.96 meters (200 feet) in diameter and 0.9-2.4 meters (3 to 8 feet) deep with rock size varying from 2.5-10.2 cm (1 to The design of a TF system for wastewater also 4 inches). Most rock media provide approximately includes a distribution system. Rotary hydraulic 149 m2/m3 (15 sq ft/cu ft) of surface area and less distribution is usually standard for this process, but than 40 percent void space. Packed plastic filters fixed nozzle distributors are also being used in (bio-towers), on the other hand, are smaller in square or rectangular reactors. Overall, fixed diameter (6 to 12 meters (20 to 40 feet)) and range nozzle distributors are being limited to small in depth from 4.3 to 12.2 meters (14 to 40 feet). facilities and package plants. Recently some These filters look more like towers, with the media distributors have been equipped with motorized in various configurations (e.g., vertical flow, cross units to control their speed. Distributors can be set flow, or various random packings). Research has up to be mechanically driven at all times or during shown that cross-flow media may offer better flow stalled conditions. distribution than other media, especially at low organic loads. When comparing vertical media In addition, a TF has an underdrain system that with the 60 degree cross-flow media, the vertical collects the filtrate and solids, and also serves as a media provide a nearly equal distribution of source of air for the microorganisms on the filter. wastewater minimizing potential plugging at higher The treated wastewater and solids are piped to a settling tank where the solids are separated. Intermediate-rate filters Usually, part of the liquid from the settling chamber is recirculated to improve wetting and flushing of Intermediate rate filters can be loaded up to 64 kg 3 the filter medium, optimizing the process and BOD5/100 m -d (40 lb BOD5/1000cu ft/day). In increasing the removal rate. order to ensure good distribution and thorough blending of the filter and secondary effluent, the It is essential that sufficient air be available for the system should recirculate the trickling filter successful operation of the TF. It has been found effluent. The biological solids that slough from an that to supply air to the system, natural draft and intermediate trickling filter are not as well digested wind forces are usually sufficient if large enough as those using a low-rate filter. ventilation ports are provided at the bottom of the filter and the medium has enough void area. High-rate filters The following four basic categories of filter design High-rate filters are generally loaded at the are based on the organic loading of the trickling maximum organic loading capabilities of the filter filter. and receive total BOD5 loading ranging from 64 to 3 160 kg BOD5/100 m -d (40 to 100 lb. BOD5/1000cu Low-rate filters ft/day). Achieving a secondary quality effluent is less likely for a high-rate filter without a second- Low-rate filters are commonly used for loadings of stage process. As a result, high-rate filters are often less than 40 kilograms five day biochemical oxygen used with combined processes. demand (BOD5)/100 meters cubed per day (25 lb BOD5/1000cu ft/day). These systems have fewer Roughing Filters problems than other filters with regards to filter flies, odors, and medium plugging because of the Roughing filters are designed to allow a significant lower loading rate. Low-rate filters with a rock amount of soluble BOD to bleed through the medium range in depth from 0.9 to 2.4 meters (3- trickling filter. Filters of this type generally have a 8 ft.). Most low-rate filters are circular with rotary design load ranging from 160-480 kg 3 distributors, but some filters currently in use are BOD5/100 m-d (100 to 300 lb. BOD5/1000cu rectangular. Both of these configurations are ft/day). equipped with dosing syphons or periodic pumps to provide a high wetting rate for short intervals PERFORMANCE between rest periods. A minimum wetting rate of 0.4 liters per square meter-second (0.7 gal/sq Recent efforts have been made to combine fixed- ft/min) is maintained to prevent the high rate plastic film reactors with suspended growth processes to filter medium from drying out. With a rock efficiently remove organic materials from medium, the filters tend not to be hydraulically wastewater. For example, the combination of a limited and have application limits ranging from trickling filter with an activated-sludge process has 0.01 to 0.04 liters per square meter-second (0.02 to allowed for the elimination of shock loads to the 0.06 gal/sq ft/min). more sensitive activated sludge while providing a highly polished effluent that could not be achieved The sloughed solids from a low-rate filter are by a trickling filter alone. Table 1 shows the BOD5 generally well-digested and as a result these filters removal rates for the four filter types discussed. yield less solids than higher rate filters. Secondary quality effluent is readily achievable if the low-rate Although the TF process is generally reliable, there trickling filter design incorporates filter media with is still potential for operational problems. Some of bioflocculation capabilities or good secondary the common problems are attributed to increased clarification. growth of biofilm, improper design, changing wastewater characteristics, or equipment failure. Some common problems with TF function are discussed in the Operation and Maintenance Filter Flies (Psychoda) section. Potential Cause Inadequate filter media moisture. Remedy: Increase hydraulic loading; unplug spray TABLE 1 BOD5 REMOVAL RATES FOR orifices or nozzles; use orifice opening at end of VARIOUS FILTER TYPES rotating distributor arms to spray filter walls; flood filter for several hours each week during fly season; maintain 1-2 mg/L residual chlorine on the filter for Filter Type BOD Removal (%) 5 several hours. Low Rate 80 - 90 Intermediate Rate 50 - 70 Potential Cause: Poor housekeeping. Remedy: Mow area surrounding filter and remove High Rate 65 - 85 weeds and shrubs. Roughing Filter 40 - 65 Source: Environmental Engineers Handbook, 1997. Icing Potential Cause: Low temperature of wastewater. OPERATION AND MAINTENANCE Remedy: Decrease recirculation; use high-pressure stream of water to remove ice from orifices, Disagreeable Odors from Filter nozzles, and distributor arms; reduce number of filters in service as long as effluent limits can still Potential Cause: Excessive organic load causing be met; reduce retention time in pretreatment and anaerobic decomposition in filter.